Effect of Flow Rates on the Localized Corrosion Behavior of 304 Stainless Steelin Ozonated 0.5N NaCl

摘要

The effect of flow on 304 stainless steel in a solution of 0.5 Normal NaClcontaining 0, 0.02, and 1.2 mg/liter dissolved ozone was studied at room temperature. Flow rates ranging from 0.3 to 2.0 m/s were simulated by the use of a rotating cylinder electrode. Corrosion potentials and net current densities were measured and values of breakdown and repassivation potentials were derived from cyclic polarization curves. It was found that the transition from laminar to turbulent flow for this system occurs between 0.25 and 0.35 m/s, based on the inverse relationship observed between the breakdown and repassivation potential, as well as the changes in the net current density. Under laminar flow conditions, increasing velocity shifted the repassivation potential in the active direction due to an increase in the driving force for pitting which resulted from the stabilization of the passive film, this stabilization was also reflected by an increase in the breakdown potential in the noble direction. Values of the net current density decreased with increasing velocity in this regime, due to a decrease in the diffusion boundary layer at the alloy surface. High ozone concentrations further deceased the magnitude of the net current density due to the limiting current density of ozone increasing with concentration. Under turbulent flow conditions, increased velocity caused the net current density to stabilize in deaerated and low ozone concentration solutions, however, solutions containing high concentrations of ozone experienced an increase in net current density to values similar to stagnant solutions.